Round, thin walled, spirally grooved metal ducts and their method of manufacture are known as evidenced by U.S. Pat. Nos. 3,435,852 and 3,621,884. Round ducts of this type have many uses, e.g., as cold or warm air duct for residential, commercial and inductrial construction. However, these round ducts, and the method and apparatus used in their manufacture, have certain disadvantages. Round duct require an excessive wall space thickness to carry the required volume of air for many applications. In a typical interior residential wall the space between studs will easily accept a 21/2 .times. 12 inches rectangular duct which provides a cross-sectional area of 30 square inches. A21/2 inches diameter round duct, about the largest round that will fit in this space, has a cross-sectional area of only about 2 square inches.
This emphasizes the need for rectangular duct. The problem is that the method and apparatus used to make the round, thin walled spirally grooved metal duct, as is apparent from the above mentioned patents, will not produce rectangular or flat oval duct. Further, it is not apparent how this apparatus and method could be modified to make rectangular duct.
This situation has forced the use of conventional straight walled galvanized metal duct in those applications where wall thickness dimension and duct cross-sectional area are critical. This type of conventional duct, usually rectangular, does not have many of the features that the round, thin walled, spirally grooved metal duct has, e.g., the ability to be bent around curves, etc. without significantly changing the cross-sectional area at the bend, light weight and low cost due to the use of aluminum and steel sheet having a thickness of less than 10 mils and usually in the range of 3-6 mils, a continuous spiral crimped seal, and the ability to be made in a continuous manner from relatively narrow strips of metal and requiring little hand labor. Thus, the need for rectangular thin walled, spirally grooved metal is clear.
Attempts have been made to reshape round, thin walled, spirally grooved metal duct into rectangular duct by external forces, but, because of the very thin wall, the duct tends to crimp rather than be reformed into the desired shape.
The object of the present invention is a rectangular duct having the desired features of the round, thin walled, spirally grooved metal ducts, a suitable method for making such a product, and a practical apparatus for use in the method.